Drug-resistant strains of HIV could become more prevalent - even developing into mini-epidemics - in San Francisco over the next five years as patients live longer, healthier lives, according to a study by researchers at UCSF and UCLA.

San Francisco public health officials emphasized that drug-resistant HIV is not a health crisis and said that while the study is interesting, they don't expect it to change how doctors treat people with HIV infections.

But the study's authors and public health officers agree that it's critical that new drug therapies continue to be developed as resistance to older treatments grows, and that funding be made available to test for resistance early so patients get appropriate care.

"Unless things change significantly, what we see is that one of the major drug classes we use to treat patients will have a rise in resistance. If that happens, that's just one less way that we can effectively treat patients," said Dr. James Kahn, an author of the study and a professor at the UCSF Positive Health Program at San Francisco General Hospital.

Drug resistance in HIV treatment emerged more than two decades ago, but as the virus evolved and became resistant to more drugs, new therapies were developed.

Resistance occurs as a drug loses its effectiveness over time, usually because the patient doesn't follow strict treatment protocols. But drug-resistant strains can be transmitted too.

Increasingly, say the UCSF and UCLA scientists, drug-resistant strains are improving their ability to transfer from person to person. According to their research, about 60 percent of the drug-resistant strains circulating in San Francisco are infectious enough that they could create mini-epidemics among HIV-positive patients.

"Drug-resistant strains are generally less infectious, but we've found they're not much less infectious," said Sally Blower, a study author and director of UCLA's Center for Biomedical Modeling. "For some of these strains, they've reached a threshold at which they're generating their own epidemic."

Part of the reason resistance is growing, say the study authors, is the aggressive way HIV is treated in developed countries like the United States. New standards recommend that people who are infected be treated early to keep them healthy. But the longer a person is on a certain drug, the greater his or her chances of developing resistance.

"What I'm concerned about is that people will use this paper either to not treat or to delay treatment," Colfax said. "Resistance should figure into clinical decisions. But you have to consider the huge benefits of early treatment for the individual and for society."

The study authors say doctors should be well-trained and prepared with backup plans to treat their patients if they develop resistance. That can be a particularly challenging task in developing countries, where access to the newest drug therapies might be limited.

"We're able to treat people with alternative medications. But within the developing world, they don't have as many treatment options," Kahn said. "Where we see huge potential for a problem is if there's no plan for the drugs that need to be used when treatment fails. And the treatment fails. It's not curative."